A fishing technique known as "deep trolling" has evolved its own special equipment. A heavy weight, often streamlined, is lowered on a cable with a winch to a point below the level at which the fish are suspected of congregating. This point is often located by modified electronic depth-finding equipment. As the cable is unwound from the reel, lowering the weight, the fish line (with bait attached) is clipped to a release device on the cable. These devices are designed so that a pull corresponding to the bite of a fish will provide enough force to pull the fish line free of the clip, leaving the fish line to the exclusive control of the fishing rod that had been in storage position up to this point in a holder at the gunwale of the boat. The behavior of the fish line makes it obvious that the bait has been struck, and the fisherman then removes the rod from the holder and manipulates it in the usual manner.
A great variety of devices are available for attachment to the downrigger cable to provide the line-release feature. Some of these involve components that are attached to the fish line, and are pulled free of the release. device at a predetermined force. Anything permanently attached to the fish line is likely to interfere with the manipulation of the rod and reel. Another problem in the use of these devices has been the notorious difference in the ways various kinds of fish will attack a bait. Some of them are quite vicious about it; and it is desirable to have a fairly substantial pull-out force, so that the hook can be properly set. Others appear to be much more gentle, and the line must be freed in response to a much smaller force. The fisherman is usually fairly well aware of the type of fish he expects, and should be able to adjust the release device accordingly. When the device has been properly "adjusted", it should be attachable easily to the cable. This is particularly desirable when a series of devices are to be attached to the line to place baits at different levels. It must be kept in mind that the fish line is not fed through the release devices, but is engaged and released laterally.